Variable focal length lens system with focus monitoring and control

ABSTRACT

A variable focal length (VFL) lens system is provided including a tunable acoustic gradient (TAG) lens and an optical focus monitoring configuration for providing a focus monitoring signal that reflects a focus state with high accuracy and without significant latency. An input illumination pattern is transmitted through the TAG lens to provide a corresponding output illumination pattern that has a size and intensity that depends on the optical power of the TAG lens. An optical focus signal detector portion includes a filtering configuration and a focus photodetector that provides a focus output signal that varies in relation to the total light energy that the focus photodetector receives, wherein the filtering configuration receives the output illumination pattern and limits the amount of included focus detection light that reaches the focus photodetector. A focus monitoring signal is provided based on the focus output signal provided by the focus photodetector.

BACKGROUND Technical Field

This disclosure relates to precision metrology using a variable focuslens, and to machine vision inspection systems and other systems inwhich a variable focal length lens may periodically modulate a focusposition.

Description of the Related Art

Precision machine vision inspection systems (or “vision systems” forshort) may be used for measuring and inspecting objects. Such systemsmay include a computer, camera, optical system, and a stage that movesto allow workpiece traversal. One exemplary system, characterized as ageneral-purpose “off-line” precision vision system, is the QUICK VISION®series of PC-based vision systems and QVPAK® software available fromMitutoyo America Corporation (MAC), located in Aurora, Ill. The featuresand operation of the QUICK VISION® series of vision systems and theQVPAK® software are generally described, for example, in the QVPAK 3DCNC Vision Measuring Machine User's Guide, published January 2003, whichis hereby incorporated herein by reference in its entirety. This type ofsystem uses a microscope-type optical system and moves the stage toprovide inspection images of small or large workpieces at variousmagnifications.

In various applications, for high throughput it is desirable to performhigh speed measurements in either stationary or non-stop movinginspection systems. With respect to Z-height measurements, which aregenerally based on the “best focus” height determination, the speed atwhich the Z-height measurements can be performed may be limited by theZ-height focus position adjustment or motion speed. However, someinnovative variable focus lenses are able to change focus at a very highrate, and determining their actual focus position with high accuracy, ata rate commensurate with their rate of focus variation, has provedproblematic. Improved Z-height measurement accuracy and speed is neededfor various high-speed variable focus lenses used for high-speedprecision inspection operations.

BRIEF SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

A variable focal length (VFL) lens system is provided including atunable acoustic gradient (TAG) lens and an optical focus monitoringconfiguration for providing a focus monitoring signal that reflects thefocus state of the VFL lens system with high accuracy and withoutsignificant latency. The TAG lens is operated to periodically modulateits optical power over a range of optical powers at an operatingfrequency. The optical focus monitoring configuration includes amonitoring light source and an optical focus signal detector portion.The monitoring light source is configured to input a focus detectionlight into the TAG lens during the periodic modulation. In variousembodiments, the input focus detection light is configured to provide aninput amount of light energy distributed in an input illuminationpattern having an approximately constant size. In some embodiments, theinput amount of light energy is approximately constant. At least acentral portion of the input illumination pattern is transmitted throughthe TAG lens during the periodic modulation to provide a correspondingoutput illumination pattern from the TAG lens, wherein the outputillumination pattern has a size and intensity that depends on theoptical power of the TAG lens. The optical focus signal detector portionis positioned at an approximately constant distance from the TAG lens toreceive focus detection light included in the output illuminationpattern output from the TAG lens. The optical focus signal detectorportion includes a filtering configuration and a focus photodetectorthat provides a focus output signal that varies in relation to the totallight energy that the focus photodetector receives, wherein thefiltering configuration receives the output illumination pattern andlimits the amount of included focus detection light that reaches thefocus photodetector. A focus monitoring signal is provided based on thefocus output signal provided by the focus photodetector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing various typical components of ageneral-purpose precision machine vision inspection system;

FIG. 2 is a block diagram of a control system portion and a visioncomponents portion of a vision system similar to that of FIG. 1 andincluding features disclosed herein;

FIG. 3 is a schematic diagram of a variable focal length lens systemincluding an optical focus monitoring portion that may be operatedaccording to principles disclosed herein;

FIGS. 4A and 4B are diagrams of an optical focus monitoring portionincluding a first exemplary implementation of an input illuminationpattern;

FIGS. 5A-5D are diagrams of an optical focus monitoring portionincluding a second exemplary implementation of an input illuminationpattern and illustrating different configurations of a spatiallyfiltering aperture or mask that may be utilized in variousimplementations;

FIG. 6 is a diagram of an optical focus monitoring portion including anormalization portion;

FIG. 7 is a diagram of a graph illustrating relationships between afocus monitoring signal and a Z-height (focus distance) for variousoptical focus monitoring portions;

FIG. 8 is a block diagram of a first exemplary implementation of adetector signal processing portion;

FIG. 9 is a block diagram of a second exemplary implementation of adetector signal processing portion; and

FIG. 10 is a flow diagram illustrating one exemplary implementation of aroutine for operating a variable focal length lens system.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one exemplary machine vision inspectionsystem 10 usable in accordance with principles disclosed herein. Thevision system 10 includes a vision measuring machine 12 operablyconnected to exchange data and control signals with a controllingcomputer system 14, a monitor or display 16, a printer 18, a joystick22, a keyboard 24, and a mouse 26. The monitor or display 16 may displaya user interface for controlling and/or programming the vision system10. A touchscreen tablet or the like may be substituted for or augmentany or all of these components.

More generally, the controlling computer system 14 may comprise orconsist of any computing system or device, and/or distributed computingenvironment, and may include one or more processors that executesoftware to perform the functions described herein. Processors includeprogrammable general- or special-purpose microprocessors, controllers,application-specific integrated circuits (ASICs), programmable logicdevices (PLDs), or a combination thereof. Software may be stored inrandom-access memory (RAM), read-only memory (ROM), flash memory, or thelike, or a combination thereof. Software may also be stored inoptical-based disks, flash memory devices, or any other type ofnon-volatile storage medium for storing data. Software may include oneor more program modules that include routines, programs, objects,components, data structures, and so on that perform particular tasks orimplement particular abstract data types. The functionality of theprogram modules may be combined or distributed across multiple computingsystems or devices and accessed via service calls, either in a wired orwireless configuration.

The vision measuring machine 12 includes a moveable workpiece stage 32and an optical imaging system 34 that may include a zoom lens orinterchangeable lenses. The zoom lens or interchangeable lensesgenerally provide various magnifications (e.g., 0.5× to 100×). Similarvision systems are described in commonly assigned U.S. Pat. Nos.7,324,682; 7,454,053; 8,111,905; and 8,111,938, each of which is herebyincorporated herein by reference in its entirety.

FIG. 2 is a block diagram of a control system portion 120 and a visioncomponents portion 200 of a vision system 100 similar to the visionsystem of FIG. 1, including features as described herein. The controlsystem portion 120 is utilized to control the vision components portion200. The vision components portion 200 includes an optical assemblyportion 205, light sources 220, 230, 240 and 243, and a workpiece stage210 that may have a central transparent portion 212. The workpiece stage210 is controllably movable along x- and y-axes that lie in a plane thatis generally parallel to the surface of the stage where a workpiece 20may be positioned.

The optical assembly portion 205 may include camera/detector 260 (e.g.,a camera portion, and/or optionally a confocal optical focus detector,or the like), a variable focal length (VFL) lens 270, a detectorconfiguration 277, and may also include an interchangeable objectivelens 250 and a turret lens assembly 280 having lenses 286 and 288.Alternatively to the turret lens assembly, a fixed or manuallyinterchangeable magnification-altering lens, or a zoom lensconfiguration, or the like, may be included.

In various implementations, the optical assembly portion 205 iscontrollably movable along a z-axis that is generally orthogonal to thex- and y-axes by using a controllable motor 294 that drives an actuatorto move the optical assembly portion 205 along the z-axis to change thefocus of an image. The controllable motor 294 is connected to aninput/output interface 130 via a signal line 296. As will be describedin more detail below, the VFL lens 270 may also be operated toperiodically modulate a focus position. A workpiece 20, or plurality ofworkpieces 20, to be imaged is/are on the workpiece stage 210 whichmoves (e.g., in the x- and y-axes directions) relative to the opticalassembly portion 205, such that the imaged area moves between locationson the workpiece(s) 20.

One or more of a stage light 220, a coaxial light 230, and a surfacelight 240 (e.g., a ring light), connected to the control system portion120 through signal lines or busses 221, 231 and 241, may emit sourcelight 222, 232, and/or 242, respectively, to illuminate the workpiece orworkpieces 20, according to known principles. In FIG. 2, the sourcelight 232 is reflected by a reflecting surface 290 to illuminate theworkpiece 20. The source light is reflected or transmitted as workpiecelight 255, (e.g., as used for imaging) which passes through theinterchangeable objective lens 250, the turret lens assembly 280 and theVFL lens 270 to the camera/detector 260. In various implementations, thecamera/detector 260 may output image data and/or other signals on asignal line or bus 262 to the control system portion 120. The controlsystem portion 120 may rotate the turret lens assembly 280 about an axis284 to select a turret lens magnification as controlled through a signalline or bus 281. As will be described in more detail below, in variousimplementations the light source 230 (or other light source) may be acontrollable strobe light source that is operably connected to andcontrolled (e.g., through the signal line or bus 231) by a strobecontroller (e.g., in the controller 125 and/or lighting controlinterface 133, etc.) A focus monitoring signal may be input to thestrobe controller and the strobe controller may control a strobe timingof the controllable strobe light source at least in part based on thefocus monitoring signal.

As will be described in more detail below with respect to FIGS. 3 and 4,a light source 243 may emit a focus detection light 245 (e.g., of adesired wavelength that does not interfere with imaging). The focusdetection light 245 may be reflected by a reflecting surface 246 to passthrough the VFL lens 270 and further reflected by a reflecting surface246′ toward a detector configuration 277. The detector configuration 277may input the focus detection light 245 and output signal data (e.g.,including a focus output signal and/or a focus monitoring signal, etc.)on a signal line or bus 278′ to a detector signal processing portion 278of the control system portion 120. The light source 243 may be connectedto the control system portion 120 through a signal line or bus 244.

As shown in FIG. 2, in various exemplary implementations, the controlsystem portion 120 includes a controller 125, the input/output interface130, a memory 140, a workpiece program generator and executor 170, and apower supply portion 190. Each of these components, and additionalcomponents described below, may be interconnected by direct connectionsor by one or more data/control busses and/or application programminginterfaces. The input/output interface 130 includes an imaging controlinterface 131, a motion control interface 132, and a lighting controlinterface 133. The motion control interface 132 may include a positioncontrol element 132 a, and a speed/acceleration control element 132 b,although such elements may be merged and/or indistinguishable. Thelighting control interface 133 may include lighting control elements 133a, 133 n, and 133 fl that control, for example, the selection, power,on/off switch, and strobe pulse timing, if applicable, for the variouscorresponding light sources of the vision system 100.

In accordance with the principles disclosed herein, the input/outputinterface 130 may further include a lens controller/interface 271, afocus signal processing portion 275 and a detector signal processingportion 278, as will be described in more detail below with respect toFIGS. 3-6. Briefly, in one implementation, the lens controller/interface271 may include a lens controller including a lens focus operatingcircuit and/or routine, or the like. The lens controller/interface 271may be configured or controlled by a user and/or an operating program,and may utilize the signal line 271′ to control the VFL lens 270 toperiodically modulate its optical power (e.g., sinusoidally) and therebyperiodically modulate a focus position of the imaging system over aplurality of focus positions along a Z-height direction at a determinedoperating frequency. The periodically modulated VFL lens optical powerdefines a periodic focus modulation. As will be described in more detailbelow with respect to FIG. 8, in one implementation the detector signalprocessing portion 278 may include a latching circuit wherein a strobetiming (e.g., for the light source 230) triggers latching of acorresponding focus monitoring signal value that is indicative of aZ-height at a corresponding image exposure timing determined by thestrobe timing. As will be described in more detail below with respect toFIG. 9, in one implementation the detector signal processing portion 278may include a comparator circuit that inputs a focus monitoring signaland a reference signal related to a Z-height in order to trigger acontrollable strobe light source (e.g., the light source 230) at aZ-height that the reference signal is related to. In variousimplementations, the focus signal processing portion 275 may also oralternatively provide a focus position indicating signal and/ordetermine a focus monitoring signal value corresponding to when signaldata from the camera/detector 260 (e.g., including a camera portion, aconfocal optical detector, etc.) indicates that an imaged workpiecesurface region is at a focus position.

In various implementations, the imaging control interface 131 and/orlens controller/interface 271 may further include an extended depth offield mode, as described in more detail in copending and commonlyassigned U.S. Patent Publication No. 2015/0145980, which is herebyincorporated herein by reference in its entirety. Other systems andmethods including VFL lenses are described in copending and commonlyassigned U.S. patent application Ser. No. 14/795,409, entitled“Adaptable Operating Frequency of a Variable Focal Length Lens in anAdjustable Magnification Optical System”, filed on Jul. 9, 2015, U.S.patent application Ser. No. 14/841,051, entitled “Multi-Level ImageFocus Using a Tunable Lens in a Machine Vision Inspection System”, filedon Aug. 31, 2015, and in copending and commonly assigned U.S. patentapplication Ser. No. 14/854,624, entitled “Chromatic AberrationCorrection in Imaging System Including Variable Focal Length Lens”,filed on Sep. 15, 2015, each of which is hereby incorporated herein byreference in its entirety.

The memory 140 may include an image file memory portion 141, anedge-detection memory portion 140 ed, a workpiece inspection programmemory portion 142, and a video tool portion 143. The video tool portion143 includes video tool portion 143 a and other video tool portions(e.g., 143 n) that determine the GUI, image-processing operation, etc.,for each of the corresponding video tools, and a region of interest(ROI) generator 143 roi that supports operations in various video tools.An autofocus video tool 143 af may determine the GUI, image-processingoperation, etc., for certain focus height measurement operations. Theautofocus video tool 143 af may additionally include a high-speed focusheight tool, as described in more detail in copending and commonlyassigned U.S. Patent Publication No. 2014/0368726, which is herebyincorporated herein by reference in its entirety. In variousimplementations, the optical focus monitoring that is described herein(e.g., including utilization of the detector configuration 277, thedetector signal processing portion 278, and/or other related elements)may be utilized in conjunction with, or otherwise included in, one ormore of the video tools.

In the context of this disclosure, and as is known by one of ordinaryskill in the art, the term “video tool” generally refers to automatic orprogrammed operations that a machine vision user can implement through arelatively simple user interface, without creating the step-by-stepsequence of operations included in the video tool. For example, a videotool may include a complex pre-programmed set of image-processingoperations that are applied and customized in a particular instance byadjusting a few governing variables or parameters. In addition to theunderlying operations and computations, the video tool comprises theuser interface that allows the user to adjust those parameters for aparticular instance of the video tool. The user interface features aresometimes referred to as the video tool with the underlying operationsbeing included implicitly.

The signal line 262 from the camera/detector 260, the signal line 271′from the VFL lens 270, the signal line 278′ from the detectorconfiguration 277 and the signal line 296 from the controllable motor294 are connected to the input/output interface 130. In addition tocarrying image data, the signal line 262 may carry a signal from thecontroller 125 that initiates certain processes (e.g., imageacquisition, confocal brightness measurement, etc.)

User interface display devices 136 (e.g., the display 16 of FIG. 1) andinput devices 138 (e.g., the joystick 22, keyboard 24, and mouse 26 ofFIG. 1) may also be connected to the input/output interface 130. Thedisplay devices 136 may display user interface features associated withthe lens controller/interface 271, the focus signal processing portion275, the detector signal processing portion 278, etc., in someembodiments.

FIG. 3 is a schematic diagram of a VFL lens system 300 that may beadapted to a vision system and operated according to the principlesdisclosed herein. It will be appreciated that certain numberedcomponents 3XX of FIG. 3 may correspond to and/or have similaroperations as similarly numbered components 2XX of FIG. 2, except asotherwise described below. As shown in FIG. 3, the VFL lens system 300includes light sources 330 and 343, an objective lens 350, a tube lens351, a relay lens 352, a VFL lens 370, a relay lens 386, a lenscontroller 371, a camera/detector 360, an optical focus monitoringportion 376, and a focus monitoring signal (FMS) calibration portion373. In various implementations, the various components may beinterconnected by direct connections or one or more data/control busses(e.g., a system signal and control bus 395) and/or applicationprogramming interfaces.

In operation, in the implementation shown in FIG. 3, the light source330 may be a “coaxial” or other light source configured to emit thesource light 332 (e.g., with strobed or continuous illumination) along apath including a partial mirror 390 and through the objective lens 350to a surface of a workpiece 320, wherein the objective lens 350 receivesthe workpiece light 355 that is focused at a focus position FP proximateto the workpiece 320, and outputs the workpiece light 355 to the tubelens 351. The tube lens 351 receives the workpiece light 355 and outputsit to the relay lens 352. In other implementations, analogous lightsources may illuminate the field of view in a non-coaxial manner, forexample a ring light source may illuminate the field of view. In variousimplementations, the objective lens 350 may be an interchangeableobjective lens and the tube lens 351 may be included as part of a turretlens assembly (e.g., similar to the interchangeable objective lens 250and the turret lens assembly 280 of FIG. 2). In various implementations,any of the other lenses referenced herein may be formed from or operatein conjunction with individual lenses, compound lenses, etc.

The relay lens 352 receives the workpiece light 355 and outputs it tothe VFL lens 370. The VFL lens 370 receives the workpiece light 355 andoutputs it to the relay lens 386. The relay lens 386 receives theworkpiece light 355 and outputs it to the camera/detector 360. Invarious implementations, the camera/detector 360 may capture an image ofthe workpiece 320 during an image exposure period, and may provide theimage data to a control system portion. In various implementations, thecamera/detector 360 may also or alternatively include a confocal opticaldetector, or the like.

In various implementations, the optional focus signal processing portion375 may input data from the camera/detector 360 and may provide data orsignals that are utilized to determine when an imaged surface region(e.g., of the workpiece 320) is at a focus position. For example, in animplementation where the camera/detector 360 includes a camera, one ormore images acquired by the camera (e.g., an image stack), may beanalyzed using a known “maximum contrast” analysis to determine when animaged surface region of the workpiece 320 is at a focus position.Exemplary techniques for such an analysis are taught in U.S. Pat. Nos.6,542,180 and 9,060,117, each of which is commonly assigned and herebyincorporated herein by reference in its entirety. In an implementationwhere the camera/detector 360 includes a confocal optical detector, atleast some of the signal data may correspond to a sensed confocalbrightness. In such an implementation, the optional focus signalprocessing portion 375 may be utilized during the periodic modulation ofthe optical power of the VFL lens 370 to determine when a maximumconfocal brightness occurs, as corresponding to a focus position of theworkpiece 320.

The VFL lens 370 is electronically controllable to vary the focusposition FP of the imaging system (e.g., during or between one or moreimage exposures, etc.). The focus position FP may be moved within arange R bound by a focus position FP1 and a focus position FP2. It willbe appreciated that in various implementations, the range R may beselected by a user or may result from design parameters or may otherwisebe automatically determined. In general, with respect to the example ofFIG. 3, it will be appreciated that certain of the illustrateddimensions may not be to scale. For example, the VFL lens 370 may havedifferent proportional dimensions than those illustrated (e.g., may beless wide and up to 50 mm long or longer for certain applications inorder to provide a desired amount of lensing power, etc.).

In various implementations, a vision system may comprise a controlsystem (e.g., the control system portion 120 of FIG. 2) that isconfigurable to operate in conjunction with a lens controller 371 or tootherwise control the VFL lens 370 to periodically modulate a focusposition of the VFL lens system 300. In some implementations, the VFLlens 370 may rapidly adjust or modulate the focus position periodically.In various implementations, the lens controller 371 may operate to drivethe VFL lens 370 (e.g., a TAG lens) at a resonant frequency in order toperiodically modulate the VFL lens optical power over the range ofoptical powers at the operating frequency. As will be described in moredetail below, a focus monitoring signal may be input to the lenscontroller 371 (e.g., as a feedback signal) and used to adjust thecontrol of the VFL lens 370. In various implementations, the adjustmentof the control of the VFL lens 370 may include adjusting at least one ofthe amplitude, frequency, or phase of the periodic modulation of the VFLlens 370.

In one example implementation, the range R over which the focus positionFP may be moved may be approximately 10 mm (e.g., for a 1× objectivelens 350). In various implementations, the VFL lens 370 isadvantageously chosen such that it does not require any macroscopicmechanical adjustments in the imaging system and/or adjustment of thedistance between the objective lens 350 and the workpiece 320 in orderto change the focus position FP.

In various implementations, the VFL lens 370 may be a tunable acousticgradient index of refraction (“TAG”) lens. A tunable acoustic gradientindex of refraction lens is a high-speed VFL lens that uses sound wavesin a fluid medium to modulate a focus position and may periodicallysweep a range of focal lengths at a frequency of several hundred kHz.Such a lens may be understood by the teachings of the article,“High-speed varifocal imaging with a tunable acoustic gradient index ofrefraction lens” (Optics Letters, Vol. 33, No. 18, Sep. 15, 2008), whichis hereby incorporated herein by reference in its entirety. Tunableacoustic gradient index lenses and related controllable signalgenerators are available, for example, from TAG Optics, Inc., ofPrinceton, N.J. The Model TL2.B.xxx series lenses, for example, arecapable of modulation up to approximately 600 KHz.

The VFL lens 370 may be driven by the lens controller 371, which maygenerate a signal to operate the VFL lens 370. In one embodiment, thelens controller 371 may be a commercial controllable signal generator.In some implementations, the lens controller 371 may be configured orcontrolled by a user and/or an operating program (e.g., through the lenscontroller/interface 271, as outlined previously with respect to FIG.2). In some implementations, the lens controller 371 may control the VFLlens 370 to periodically modulate its optical power (e.g., sinusoidally)and thereby periodically modulate a focus position of the imaging systemover a plurality of focus positions along a Z-height direction at a highoperating frequency (e.g., as high as 400 kHz, or 600 kHz, etc.),although slower focus position modulation frequencies may be desirablein various implementations and/or applications. For example, in variousimplementations a periodic modulation of 300 Hz, or 3 kHz, or 70 kHz, or250 kHz, or the like, may be used. In implementations where slowerperiodic focus position adjustments are used, the VFL lens 370 maycomprise a controllable fluid lens, or the like. In variousimplementations, the periodically modulated VFL lens optical power maydefine a periodic focus modulation.

In various implementations, the lens controller 371 may include a drivesignal generator portion 372. The drive signal generator portion 372 mayoperate (e.g., in conjunction with a timing clock 372′) to provide aperiodic drive signal to a high speed VFL such as a TAG lens. In variousimplementations, the periodic signal may have the same operatingfrequency as the periodically modulated VFL lens optical power, and in aprior art TAG lens the approximate focus height or Z-height of a TAGlens has been determined based on a concurrent state of the drivesignal. However, due to the high focus variation frequency and otheroperating characteristics of a TAG lens, the drive signal may beslightly out of phase with the actual focus height or Z-height variationof the TAG lens, leading to Z-height measurement errors and/or the needfor complex and error compensation schemes such as described incopending and commonly assigned U.S. patent application Ser. No.15/145,682, entitled “Phase Difference Calibration In A Variable FocalLength Lens System”, filed on May 3, 2016, which is hereby incorporatedherein by reference in its entirety. The principles disclosed herein maybe used to overcome deficiencies in the prior art, and/or eliminate theneed for complex error compensation schemes, in order to provideprecision Z-height measurements for high speed VFL's such as a TAG lens.

According to principles disclosed herein, a focus monitoring signal maybe determined which is directly indicative of the periodic focusmodulation, approximately in real time, as will be described in moredetail below. In various implementations, the Z-height versus focusmonitoring signal calibration portion 373 may provide a first Z-heightversus focus monitoring signal value characterization that relatesrespective Z-heights to respective focus monitoring signal values.Generally speaking, the Z-height versus focus monitoring signalcalibration portion 373 comprises recorded calibration data. As such,its representation in FIG. 3 as a separate element is only schematic,and not limiting. The associated recorded calibration data could bemerged with and/or indistinguishable from the lens controller 371, orthe optical focus monitoring portion 376, or a host computer systemconnected to the system signal and control bus 395, in variousembodiments.

As will be described below with respect to FIG. 4, the optical focusmonitoring portion 376 may input focus detection light 345 that haspassed through the VFL lens 370 and may produce a focus output signal(e.g., a focus output signal from a photodetector). In variousimplementations, a focus monitoring signal may be provided based on thefocus output signal. For example, in one implementation, the focusoutput signal may be provided directly as the focus monitoring signal.As another example, in an alternative implementation, the focusmonitoring signal may be produced based on further signal processing ofthe focus output signal.

As will be described in more detail below, in various implementationsthe VFL lens system 300 may further include a pattern generator 343Gthat in combination with the light source 343 generates an inputillumination pattern PATin that is input into the VFL lens 370 (e.g., aTAG lens) and that results in an output illumination pattern PATout fromthe VFL lens 370. In various implementations, an optical path includingthe VFL lens 370 may further include a first beamsplitter 346 and asecond beamsplitter 346′. The first beamsplitter 346 is located betweenthe objective lens 350 and the VFL lens 370 and receives focus detectionlight 345 from the monitoring light source 343 and directs at least someof the focus detection light 345 to pass through the VFL lens 370. Thesecond beamsplitter 346′ is located between the VFL lens 370 and thecamera/detector 360 and receives the output illumination pattern PAToutfrom the VFL lens 370 and directs the output illumination pattern PATouttoward the optical focus monitoring portion 376 including an opticalfocus signal detector portion (e.g., as will be described in more detailbelow with respect to FIGS. 4-6).

In the specific configuration of FIG. 3, the light source 343 incombination with the pattern generator 343G produces focus detectionlight 345 in the form of a collimated beam (e.g., in the form of theinput illumination pattern PATin), at least some of which is received bythe beamsplitter 346 and is directed to pass through the VFL lens 370.The beamsplitter 346′ receives at least some of the focus detectionlight 345 that has passed through the VFL lens 370 (e.g., in the form ofthe output illumination pattern PATout) and directs the focus detectionlight toward the optical focus monitoring portion 376. Due to thepositioning and modulating optical power of the VFL lens 370, the focusdetection light 345 may be output from the VFL lens withdivergence/convergence for which the outer beam dimensions arecorrespondingly modulated and vary between maximum outer beam paths Bmaxand minimum outer beam paths Bmin (e.g., which may cause certaindimensions of the output illumination pattern PATout to correspondinglymodulate/vary, as will be described in more detail below with respect toFIGS. 4A, 4B and 5A-5D).

In one implementation, the beamsplitters 346 and 346′ may be dichroicbeamsplitters and the focus detection light 345 from the light source343 may be of a different wavelength than the source light 332 from thelight source 330. In various implementations, the monitoring lightsource 343 may produce the focus detection light 345 consisting of afirst set of wavelengths and the imaging light source 330 may producethe source light 332 consisting of a second set of wavelengths thatexcludes the first set of wavelengths. The dichroic beamsplitters 346and 346′ may each reflect the first set of wavelengths and transmit thesecond set of wavelengths. As an example, in one specificimplementation, the light source 343 may be operated in a continuousmode and may provide collimated focus detection light 345 with awavelength of approximately Δ=735 nm, for which one or both of thedichroic beamsplitters 346 and 346′ may have characteristics such asR>720 nm and T<700 nm (e.g., so as to reflect the desired focusdetection light 345 from the light source 343 while allowing workpiecelight 355 that results from the source light 332 from the light source330 to pass through as transmitted light to the camera/detector 360,etc.).

As described above, the source light 332 from the light source 330 maybe directed toward an imaged surface region (e.g., of the workpiece 320)to produce the workpiece light 355 (e.g., that is utilized to produce animage of the imaged surface region and/or to determine when the imagedsurface region is in focus), and for which the source light 332 may havea different wavelength than the focus detection light 345 (e.g., thesource light 332 being λ<700 nm while the focus detection light 345 isλ>720 nm, etc.). In various implementations, utilization of a 735 nm LEDfor the light source 343 to produce the focus detection light 345 mayhave certain advantages (e.g., having a good match to siliconresponsivity and having little or no coherence/speckle, etc.). Asanother example where more power is needed, a 785/805 nm diode laser maybe utilized as operated below threshold, etc.

In various implementations, an imaging configuration may be designatedas including at least the objective lens 350, the VFL lens 370, and thecamera/detector 360. As noted above, the objective lens 350 inputsworkpiece light 355 from an imaged surface region of the workpiece 320in the field of view (FOV) of the imaging configuration and transmitsthe workpiece light 355 through the VFL lens 370, and thecamera/detector 360 receives the workpiece light from the VFL lens 370and provides an image focused at an imaging system focal plane having atleast one of a focus distance or Z-height relative to the imagingconfiguration. In various implementations, at least one of the focusdistance or Z-height of the imaging system focal plane is controlled bythe VFL lens optical power. In such implementations, an instantaneousvalue of focus monitoring signal that is produced by the optical focusmonitoring portion 376 may be indicative of at least one of theinstantaneous focus distance or Z-height of the imaging system focalplane. In various implementations, the focus monitoring signal and/orfocus output signal may comprise a time varying signal that isindicative of the focus state of the VFL lens 370 throughout themodulation period with high accuracy and the time varying signal may beprovided without significant latency compared to the focus state. In onespecific example configuration, the periodic modulation may correspondto a frequency of at least 50 kHz, and the time varying signal may beprovided with a latency compared to the focus state of not more than 100nanoseconds. In some embodiments, even smaller latency may be attained,for example not more than 50 nanoseconds, or 25 nanoseconds, or less.Suitable ultrafast photodetectors and associated amplification circuitsare known in the art and commercially available, for example, fromHamamatsu Corporation, San Jose, Calif., and/or Newport Corporation,Santa Clara, Calif. Such photodetectors may have a rise time on theorder of 40 picoseconds, for example. The associated latencies or signallag may thus correspond to an insignificant focus measurement error orZ-height error in an imaging system using a periodically modulated highspeed VFL lens such as a TAG lens, in that the focus change during thesmall latency period may be a small portion of the depth of field of theimaging system including the VFL lens. In various implementations, anyresidual latency may further be compensated for or otherwise accountedfor by circuitry (e.g., included in the detector signal processingportion 478 of FIGS. 8 and 9) and/or other components of the VFL lenssystem 300. It will be appreciated that according to principlesdisclosed herein, the near real-time monitoring of the actual opticalpower of the VFL lens allows Z-height measurements and/or otheroperations of the VFL lens system to be accurately performed despitevarious instabilities (e.g., lens or circuit temperature sensitivity)that adversely affected prior art methods.

In various implementations, the focus monitoring signal may be utilizedfor various purposes relative to the operations of the VFL lens system300. For example, the focus monitoring signal may be input to acontroller which may utilize the focus monitoring signal (e.g., as afeedback signal) to adjust the control of the VFL lens 370. As anotherexample, a VFL lens system may generally be configured to control theimage exposure using an image exposure timing that determines thecorresponding imaging system focal plane. The VFL lens system may beconfigured to control at least one of a timing of a controllable strobelight source that is included in VFL lens system or a timing of acontrollable image integration period of the camera portion, to providethe image exposure timing. In some embodiments, a latching circuit maybe configured to latch a focus monitoring signal value at a timecorresponding to the image exposure timing, wherein the latched focusmonitoring signal value is indicative of the focus distance or Z-heightfor the corresponding image exposure. In some embodiments, a comparatorcircuit may be configured to input the focus monitoring signal and inputa reference signal related to a desired imaging focus distance orZ-height, and output a trigger signal that controls the image exposuretiming to occur when the focus monitoring signal corresponds to thereference signal. In some embodiments, the focus monitoring signal maybe input to the controller for controlling the image exposure timing, orthe focus monitoring portion 376 may include circuitry that is utilizedto control a strobe timing of the controllable strobe light sourceand/or the image integration period of the camera portion, at least inpart based on the focus monitoring signal. Exemplary specificimplementations are described in more detail below with respect to FIG.8 and FIG. 9.

In the example of FIG. 3, the relay lenses 352 and 386 and the VFL lens370 are designated as being included in a 4f optical configuration,while the relay lens 352 and the tube lens 351 are designated as beingincluded in a Keplerian telescope configuration, and the tube lens 351and the objective lens 350 are designated as being included in amicroscope configuration. All of the illustrated configurations will beunderstood to be exemplary only, and not limiting with respect to thepresent disclosure. In various implementations, the illustrated 4foptical configuration permits placing the VFL lens 370 (e.g., which maybe a low numerical aperture (NA) device, such as a TAG lens), at thefourier plane FPL of the objective lens 350. This configuration maymaintain the telecentricity at the workpiece 320 and may minimize scalechange and image distortion (e.g., including providing constantmagnification for each Z-height of the workpiece 320 and/or focusposition FP). The Keplerian telescope configuration (e.g., including thetube lens 351 and the relay lens 352) may be included between themicroscope configuration and the 4f optical configuration, and may beconfigured to provide a desired size of the projection of the objectivelens clear aperture at the location of the VFL lens, so as to minimizeimage aberrations, etc.

FIGS. 4A and 4B are diagrams of an optical focus monitoring portion 376′including a first exemplary implementation of an input illuminationpattern PATin. The optical focus monitoring portion 376′ may beunderstood to be one implementation of the optical focus monitoringportion 376 shown in FIG. 3. The beamsplitter 346′ shown in FIG. 3 isomitted in the schematically represented optical path shown in FIG. 4A,for simplicity. In the example of FIG. 4A, the optical focus monitoringportion 376′ includes a detector configuration 477′ and a detectorsignal processing portion 478 (e.g., which may correspond to orotherwise be similar to the detector configuration 277 and the detectorsignal processing portion 278 of FIG. 2). The detector configuration477′ includes a first optical focus signal detector portion DP1 whichincludes a first monitoring lens LNS1 and a first optical detector DET1.The first optical detector DET1 includes a first focus photodetector PD1and a first filtering configuration MSK1 (e.g., a mask). As will bedescribed in more detail below with respect to FIG. 6, in an alternativeconfiguration, a second detector portion DP2 may be included as part ofa detector configuration.

As described above with respect to FIG. 3, the monitoring light source343 is configured to input the focus detection light 345 into the VFLlens 370 during the periodic modulation. In the examples of FIGS. 4A-6,the VFL lens 370 is designated as being a TAG lens. As illustrated inFIG. 4A, in some embodiments, the input focus detection light 345 isconfigured to provide an input amount of light energy Ein distributed inthe input illumination pattern PATin (e.g., as generated by the patterngenerator 343G of FIG. 3) having an approximately constant size (e.g.,as indicated by a constant diameter Din, etc.). In some embodiments, theinput amount of light energy is approximately constant. The focusdetection light 345 is at least approximately collimated in the inputillumination pattern PATin.

In various implementations, the input focus detection light 345 in theform of the input illumination pattern PATin may comprise a static beamof light. In one such implementation, the static beam of light maycomprise a solid cross-section of light that overfills a limitingaperture included in the TAG lens 370, and the limiting aperture in theTAG lens 370 may define an approximately constant size (e.g., includingthe diameter Din, etc.) of the input illumination pattern PATin. Inanother such implementation, the static beam of light may be configuredin the input illumination pattern PATin having a constant size that issmall enough that the complete input illumination pattern PATin passesthrough the TAG lens 370 to form an output illumination pattern PATout.

In various implementations, at least a central portion of the inputillumination pattern PATin is transmitted through the TAG lens 370during the periodic modulation to provide the corresponding outputillumination pattern PATout from the TAG lens 370, wherein the outputillumination pattern PATout has a size and intensity that depends on theoptical power of the TAG lens 370. For example, as illustrated in FIG.4B, the output illumination pattern PATout may have a size thatmodulates/varies between a maximum diameter Dout_max and a minimumdiameter Dout_min. This variation is due at least in part due to thefact that, as a result of the positioning and modulating optical powerof the TAG lens 370, the focus detection light 345 is output from theTAG lens with divergence/convergence for which the outer beam dimensionsare correspondingly modulated and vary between maximum outer beam pathlimits Bmax and minimum outer beam path limits Bmin.

The optical focus signal detector portion DP1 is positioned at anapproximately constant distance from the TAG lens 370 to receive thefocus detection light 345 included in the output illumination patternPATout that is output from the TAG lens 370. The first monitoring lensLNS1 focuses focus detection light toward the first filteringconfiguration MSK1 and the first focus photodetector PD1. In variousimplementations, the first filtering configuration MSK1 and/or the firstfocus photodetector PD1 may be positioned at or near the best-focus ofthe first monitoring lens LNS1. In various implementations, the firstmonitoring lens LNS1 inputs the output illumination pattern PATout andtransmits it to the filtering configuration MSK1 with a reduced size.The focus photodetector PD1 provides a focus output signal 478A (e.g.,corresponding to the signal line/bus 278′ of FIG. 2) that varies inrelation to the total light energy that the focus photodetector PD1receives, wherein the filtering configuration MSK1 receives the outputillumination pattern PATout and limits the amount of included focusdetection light 345 that reaches the focus photodetector PD1.

In the example of FIGS. 4A and 4B, the input illumination pattern PATincomprises a solid pattern and is configured such that the correspondingoutput illumination pattern PATout that is output from the TAG lenscomprises a solid pattern. The filtering configuration MSK1 comprises aspatial filtering aperture AP1 that is defined by the limits of thefocus photodetector PD1 (e.g., so that the amount of the solid patternPATout reaching the focus photodetector PD1 is of a specified sizerelative to the operable area of the focus photodetector PD1), and thesolid pattern PATout overfills the aperture AP1 and/or focusphotodetector PD1 at all times during the periodic modulation.

A focus monitoring signal is provided based on the focus output signal478A provided by the focus photodetector PD1. In one implementation, thefocus output signal 478A may correspond to and be provided directly asan amplified focus monitoring signal. In an alternative implementation,the focus output signal 478A may undergo additional signal processingand/or otherwise be modified (e.g., by known linearization and/ornormalization circuit techniques, for example) and theprocessed/modified signal that is based on the first focus output signal478A may be provided as a focus monitoring signal. In someimplementations, as will be described in more detail below with respectto FIGS. 8 and 9, a detector signal processing portion 478 may input thefirst focus output signal 478A and processes it (e.g., in combinationwith other signals) to output an output signal 479 (e.g., as utilized totrigger a controllable strobe light source, or to determine a Z-heightcorresponding to when a controllable strobe light source was triggered,etc.).

In various implementations, the focus photodetector PD1 may be ahigh-speed photodetector that is utilized for accurately monitoring therapidly changing optical power of the TAG lens 370 in real time. Forexample, in certain implementations the optical power of the VFL lens370 may be modulated at rates as high as 50 kHz, 70 kHz, or 250 kHz, or400 kHz, etc., for which a high-speed focus photodetector (e.g., such aspreviously outlined herein) may be required for accurate monitoring withminimal latency. In some implementations, the focus photodetector PD1may be a high-speed, reverse-biased, silicon photodiode (SiPD) using atransimpedance amplifier. An example of devices and circuits that may beutilized in such configurations are described in U.S. Pat. Nos.4,029,976; 8,907,729; and 6,064,507, for example, each of which ishereby incorporated by reference herein in its entirety.

FIGS. 5A-5D are diagrams of an optical focus monitoring portion 376″including a second exemplary implementation of an input illuminationpattern PATin′ and illustrating different configurations of spatiallyfiltering apertures AP1′-AP1′″ and corresponding filteringconfigurations MSK1′-MSK1′″ that may be utilized in variousimplementations. Various elements of FIGS. 5A-5D may be similar oridentical to those of FIGS. 4A and 4B, as including similar or identicalreference numbers, and will be understood to operate similarly, exceptas otherwise described below. In the example of FIG. 5A, the opticalfocus monitoring portion 376″ includes a detector configuration 477″ andthe detector signal processing portion 478. The detector configuration477″ includes a first optical focus signal detector portion DP1′ whichincludes a first monitoring lens LNS1 and a first optical detectorDET1′. The first optical detector DET1′ includes a first focusphotodetector PD1 and a first filtering configuration MSK1′ (e.g., amask).

One difference for the configuration of FIG. 5A as compared to theconfiguration of FIG. 4A, is that the input illumination pattern PATin′is an annular pattern that is configured such that the correspondingoutput illumination pattern PATout′ also comprises an annular pattern(e.g., a circular ring pattern). As illustrated in FIGS. 5B-5D, theoutput illumination pattern PATout′ has a size that modulates/variesbetween a maximum diameter Dout_max with a corresponding maximum patternthickness T_Dout_max, and a minimum diameter Dout_min with acorresponding minimum pattern thickness T_(—) Dout_min.

FIGS. 5B-5D illustrate different implementations of filteringconfigurations MSK1′-MSK1′″ (e.g., masks), including different types ofspatially filtering apertures AP1′-AP1′″, respectively. As will bedescribed in more detail below with respect to FIG. 7, the differentfiltering configurations MSK1′-MSK1′″ result in different types ofrelationships between a focus monitoring signal and a determinedZ-height (focus distance). As shown in FIGS. 5B and 5C, for each of thefiltering configurations MSK1′ and MSK1″, the spatially filteringapertures AP1′ and AP1″ of the masks have different shapes. Morespecifically, the spatially filtering aperture AP1′ has a constant widthWap′, while the spatially filtering aperture AP1″ is curved and has awidth Wap″ defined by a function kDout̂2. Each of the masks of thefiltering configurations MSK1′ and MSK1″ blocks a blocked portion of theoutput illumination pattern PATout at all times during the periodicmodulation. Each of the spatially filtering apertures AP1 and AP1″ alsotransmits a transmitted portion of the output illumination patternPATout at all times. In addition, each of the spatially filteringapertures AP1′ and AP1″ is shaped such that the ratio of the transmittedportion to the blocked portion of the output illumination pattern PAToutvaries depending on the size of the output illumination pattern PATout.This is in contrast, for example, to a “pie-shaped” spatially filteringaperture, for which the ratio of the transmitted portion to the blockedportion would remain constant. As will be described in more detail belowwith respect to FIG. 7, the shape of the spatially filtering apertureAP1″ (i.e., for which Wap″=kDout̂2) causes the focus output signal and/orthe focus monitoring signal to be proportional to the focus state of thevariable focal length (VFL) lens system.

As shown in FIG. 5D, the filtering configuration MSK1′″ includes a maskcomprising a density filter AP1′″ having a non-uniform density patternconfigured to attenuate the transmission of the output illuminationpattern PATout depending on the size (e.g., the diameter Dout) of theoutput illumination pattern PATout. In the example of FIG. 5D, thenon-uniform density pattern is axisymmetric and the density varies as afunction of radius within the pattern (e.g., defined as a function ofDout, such as density=kDout). The density filter AP1′″ is configured toreceive and transmit the entire output illumination pattern PATout tothe photodetector PD1 at all times during the periodic modulation,although as filtered by the non-uniform density pattern. As will bedescribed in more detail below with respect to FIG. 7, the non-uniformdensity pattern of the density filter AP1′″ (e.g., with a transmissionfunction such as density=kDout) may cause the focus output signal and/orthe focus monitoring signal to be proportional to the focus state of thevariable focal length (VFL) lens system.

FIG. 6 is a diagram of an optical focus monitoring portion 376′″including a normalization portion DP2. Certain elements of FIG. 6 aresimilar or identical to those of FIGS. 4A, 4B and 5A-5D, as includingsimilar or identical reference numbers, and will be understood tooperate similarly, except as otherwise described below. In particular,the optical focus signal detector portion DP1″ may be configuredsimilarly to any of the previously outlined embodiments of the opticalfocus signal detector portions DP1 or DP1′. In the example of FIG. 6,the optical focus monitoring portion 376 m includes a detectorconfiguration 477′″ and the detector signal processing portion 478. Thedetector configuration 477′″ includes a beamsplitter BS1, the firstoptical focus signal detector portion DP1″, and a second optical focussignal detector portion DP2 (e.g., also referenced as a normalizationsignal detector portion DP2). The first optical focus signal detectorportion DP1″ includes a first monitoring lens LNS1 and a first opticaldetector DET1″. The first optical detector DET1″ includes a first focusphotodetector PD1 and a first filtering configuration MSK1″ (e.g., amask). The normalization signal detector portion DP2 includes a secondmonitoring lens LNS2 (e.g., also referenced as a normalization lensLNS2) and a second optical detector DET2 (e.g., also referenced as anormalization optical detector DET2), which includes a second focusphotodetector PD2 (e.g., also referenced as a normalizationphotodetector PD2).

In operation, the beamsplitter BS1 (e.g., a non-polarizing 50/50beamsplitter) receives focus detection light 345 as part of the outputillumination pattern PATout (e.g., from the beamsplitter 346′ of FIG. 3)that has passed through the TAG lens 370, and transmits at least somefocus detection light as a first split output illumination patterntoward the optical focus signal detector portion DP1″, which operatesaccording to previously outlined principles. At least some focusdetection light is also directed as a second split output illuminationpattern toward the normalization signal detector portion DP2. Thenormalization signal detector portion DP2 including the normalizationlens LNS2 inputs the entire second split output illumination pattern andtransmits the entire second split output illumination pattern to thenormalization photodetector PD2 with a reduced size, wherein thenormalization photodetector PD2 provides a normalization output signal478B (e.g., corresponding to the signal line/bus 278′ of FIG. 2) thatvaries in relation to the total light energy it receives. In variousimplementations, the detector signal processing portion 478 may include,or be part of, a focus monitoring output circuit that is configured toinput the focus output signal 478A and the normalization output signal478B, and to produce a normalized focus output signal and/or anormalized focus monitoring signal, wherein variations in the focusoutput signal 478A due to variations in the total light energy includedin the output illumination pattern PATout are compensated based on thenormalization output signal 478B according to known techniques (e.g., bydividing the focus output signal 478A by the normalization output signal478B).

FIG. 7 is a diagram of a graph 700 illustrating relationships between afocus monitoring signal (e.g., as output from a focus photodetector) anda Z-height (focus distance) for various optical focus monitoringconfigurations. As illustrated in FIG. 7, a curve 710 represents theoutput from the focus photodetector PD1 when the filtering configurationMSK1 of FIG. 4B is utilized. A curve 720 represents the output from thefocus photodetector PD1 when the filtering configuration MSK1′ of FIG.5B is utilized. A curve 730 represents the output from the focusphotodetector PD1 when the filtering configuration MSK1″ of FIG. 5C isutilized or when the filtering configuration MSK1′″ of FIG. 5D isutilized. The curves 710-730 may be better understood with reference tothe following equations. In the following equations, a constant K isdesignated as representing a combined value of all constants that mayexist in the equation, such that for each equation in which one or moreconstant values are present, there will only be one generic constant Killustrated.

As a first relevant equation, the intensity i(t) in the pattern at themask plane may be represented by:

i(t)=Ein/A(t)  (Eq. 1)

where Ein is the total energy in the input pattern PATin, and A(t) isthe total area in the output pattern at the mask plane. For the signalS(t):

S(t)=I(t)*Tr  (Eq. 2)

where Tr may be the transmitted area, or alternatively the transmittedarea times a filtering coefficient. In the following equations, it isassumed that Zfocus is approximately proportional to Dout, such that:

Zfocus(t)=K(Dout(t))  (Eq. 3)

For the solid pattern and spot configuration of FIGS. 4A and 4B:

A(t)=pi*(Dout(t)/2)²  (Eq. 4)

for which if the fixed mask aperture (Tr=constant) of FIG. 4B isutilized that is always overfilled, the signal is proportional to theintensity. That is:

S(t)=K*i(t)=K[Ein/pi*(Dout(t)/2)²]  (Eq. 5)

Assuming Ein is constant (as it may be in some embodiments), andrearranging (and using K as noted above to represent any modifiedconstant of proportionality):

S(t)=K/(Dout(t)/2)²  (Eq. 6)

which results in:

Zfocus(t)=K[(1/S(t)]^(1/2)  (Eq. 7)

That is, Zfocus is approximately inversely proportional to the squareroot of the signal S(t). In various implementations, a relatedcalibration table may be provided, or a conversion may be performedanalytically.

For the annulus pattern and filtering configuration of FIG. 5B:

A(t)≅pi*[Dout(t)*T(Dout)]  (Eq. 8)

for which the filtering configuration MSK1′ utilizes the mask apertureAP1′ that has a constant width Wap′ for all values of D (or Dout). Insuch a configuration:

Tr=2*Wap′*T(Dout)  (Eq. 9)

With reference to EQUATION 2, and substituting for Tr using EQUATION 9and for i(t) using EQUATION 1:

S(t)=i(t)*Tr=Tr*i(t)=Tr*[Ein/A(t)]=Wap′*T(Dout)*[Ein/A(t)]  (Eq. 10)

and further substituting for A(t) using EQUATION 8:

S(t)=Wap′*T(Dout)Ein/[pi*Dout(t)*T(Dout)]  (Eq. 11)

Assuming Ein is constant (as it may be in some embodiments), andrearranging (and utilizing K as a modified constant of proportionalityfor Ein and pi, etc.):

S(t)=K*Wap′/Dout(t)  (Eq. 12)

In an implementation where Zfocus is approximately proportional to Dout,this results in:

Zfocus(t)=K Wap′/S(t)  (Eq. 13)

Or, since Wap′ is constant in the embodiment of FIG. 5B:

Zfocus(t)=K/S(t)  (Eq. 14)

That is, Zfocus is approximately inversely proportional to the signalS(t), as indicated by the curve 720 of FIG. 7. With respect to the aboveequations, it will be appreciated that the shape of the aperture may beutilized to influence the relationship between Zfocus and the signalS(t).

Using this concept, if Wap′=K*Dout² in EQUATION 12, then the signal isproportional to Dout, and as a result Zfocus(t) is proportional to thesignal S(t). One such configuration is illustrated in FIG. 5C (e.g.,where the aperture AP1″ flares open like a horn at increasing D suchthat it subtends a greater portion or angle of the output illuminationpattern PATout′ for larger Dout). Alternatively, in the configuration ofFIG. 5D, the filtering configuration MSK1″ uses the entire annulusoutput illumination pattern PATout′, but filters it through an“aperture” which comprises a density filter AP1′″. For such aconfiguration, instead of filtering through a mask aperture width, theoutput illumination pattern PATout′ is filtered through a mask densityfunction that is a function of Dout. This density filter functionF(Dout) is analogous to the width function Wap” discussed above inreference to FIG. 5C. That is, 360 degrees of the annulus outputillumination pattern PATout′ is always transmitted, but it istransmitted with a transmission coefficient F(Dout) that depends ondiameter, rather than by an “aperture width” that depends on diameter.By analogy with the previous equations, and because the entire outputpattern is always transmitted through the filter, for thisconfiguration:

Tr=F(Dout)*A(t)  (Eq. 15)

In this case F(Dout) defines the proportion of transmission. In suchinstances, larger F equates to more transmission. With respect toEQUATION 2, and substituting for Tr using EQUATION 15, and for i(t)using EQUATION 1:

S(t)=i(t)*Tr=Tr*i(t)=Tr*[Ein/A(t)]=F(Dout)*A(t)*[Ein/A(t)]  (Eq. 16)

using K to mean a modified constant of proportionality for Ein, etc.,simplifying EQUATION 16, and noting that Dout is a function of timeDout(t):

S(t)=K*F(Dout(t))  (Eq. 17)

That is, the output signal S(t) will generally be proportional to thevalue of the density function at any particular output pattern diameterof the output annulus pattern at that particular time. Suchconfigurations indicate that any density function may be chosen in orderto create a particular signal in relation to a particular value of Doutand/or Zfocus. For example, if:

F(Dout)=K*Dout  (Eq. 18)

and substituting EQUATION 18 into EQUATION 16:

S(t)=K*Dout(t)  (Eq. 19)

In implementations where Zfocus is at least approximately proportionalto Dout, this indicates that:

Zfocus(t)=(1/K)*S(t)  (Eq. 20)

Thus, for this particular density function for the configuration of FIG.5D, the signal is proportional to Dout, which indicates that Zfocus(t)is proportional to the signal S(t), which in various implementations maybe a convenient configuration for intuitive understanding of the systemand for signal processing, etc. It is noted that this result is similarto the result noted above for Wap″=K*Dout² for the configuration of FIG.5C.

FIG. 8 is a schematic diagram of a first exemplary implementation of adetector signal processing portion 478′ (e.g., as one implementation ofthe detector signal processing portion 478 of FIGS. 4A and 5A). As shownin FIG. 8, the detector signal processing portion 478′ includes alatching circuit 810 which receives the focus output signal 478A as afocus monitoring signal (e.g., provided by the focus photodetector PD1of FIG. 4A or 5A), and receives a latching signal 812, and provides anoutput signal 479′. In one implementation, the latching signal 812 maycorrespond to a strobe timing signal (e.g., for the light source 330)that determines a corresponding timing for imaging a surface region of aworkpiece 320 in the field of view (FOV) of the imaging configuration.In such an implementation, the strobe timing in the latching signal 812triggers in the latching circuit 810 a latching of a corresponding focusmonitoring signal value, wherein the latched focus monitoring signalvalue is provided as the output signal 479′ and is indicative of theZ-height at the corresponding image exposure timing determined by thestrobe timing. FIG. 9 is a schematic diagram of a second exemplaryimplementation of a detector signal processing portion 478″ (e.g., asone implementation of the detector signal processing portion 478 ofFIGS. 4A and 5A). As shown in FIG. 9, the detector signal processingportion 478″ includes a comparator circuit 910 which receives the focusoutput signal 478A as a focus monitoring signal (e.g., provided by thefocus photodetector PD1 of FIG. 4A or 5A), and receives a referencesignal 912, and provides an output signal 479″. In one implementation,the reference signal 912 may be related to a focus monitoring signalvalue that corresponds to a desired Z-height for imaging a surfaceregion of a workpiece 320 in the field of view (FOV) of the imagingconfiguration. For example, the reference signal 912 may be determinedfrom stored data in the Z-height versus focus monitoring signalcalibration portion 373. In operation of the comparator circuit 910, thereference signal 912 triggers a strobe timing on the output signal 479″for a controllable strobe light source (e.g., the light source 330) whenthe focus modulation of the imaging system as indicated by the focusmonitoring signal 478A is at the Z-height that the reference signal 912is related to.

FIG. 10 is a flow diagram illustrating one exemplary implementation of aroutine 1000 for operating a VFL lens system. At a block 1010, a TAGlens that is part of a VFL system is operated to periodically modulatethe TAG lens optical power over a range of optical powers at anoperating frequency. At a block 1020, a focus detection light is inputinto the TAG lens during the periodic modulation, wherein the inputfocus detection light is configured to provide an input amount of lightenergy distributed in an input illumination pattern having anapproximately constant size. In some embodiments, the input amount oflight energy is approximately constant. At a block 1030, at least acentral portion of the input illumination pattern is transmitted throughthe TAG lens during the periodic modulation to provide a correspondingoutput illumination pattern from the TAG lens, the output illuminationpattern having a size and intensity that depends on the optical power ofthe TAG lens.

At a block 1040, focus detection light included in the outputillumination pattern is received using an optical focus signal detectorportion positioned at an approximately constant distance from the TAGlens. In various implementations, the optical focus signal detectorportion comprises a filtering configuration and a photodetector thatprovides a focus output signal that varies in relation to the totallight energy it receives, wherein the filtering configuration receivesthe output illumination pattern and limits the amount of included focusdetection light that reaches the photodetector. At a block 1050, a focusmonitoring signal is provided based on the focus output signal providedby the photodetector, wherein the focus monitoring signal reflects thefocus state of the VFL lens system with high accuracy and withoutsignificant latency.

While preferred implementations of the present disclosure have beenillustrated and described, numerous variations in the illustrated anddescribed arrangements of features and sequences of operations will beapparent to one skilled in the art based on this disclosure. Forexample, in a number of the examples and embodiments above, theoperation is simpler to explain and understand assuming that anapproximately constant amount of light energy is distributed in an inputillumination pattern having an approximately constant size, in whichcase the focus output signal (e.g., in an amplified form) may be used asthe focus monitoring signal in some embodiments. However, suchembodiments are exemplary only, and not limiting. In particular, theembodiment shown in FIG. 6 discloses providing and using a normalizationoutput signal. In such an embodiment, the focus output signal and thenormalization output signal may be used in combination to provide a“normalized focus output signal” that may be used as a reliable focusmonitoring signal. It is not necessary that the input light energy isconstant in such an embodiment. In another alternative embodiment, thefocus output signal (or a normalized focus output signal) may be used asa feedback to regulate the light source 330 to keep the focus outputsignal (or a normalized focus output signal) constant. The light sourcecan typically be regulated at a very high rate (e.g., 5-20 MHzcorrection rates). In such an embodiment, a focus monitoring signal maybe provided based on the level of the light source driving signal thatis used to maintain the constant focus output signal.

The foregoing examples illustrate that various alternative forms may beused to implement the principles disclosed herein. In addition, thevarious implementations described above can be combined to providefurther implementations. All of the U.S. patents and U.S. patentapplications referred to in this specification are incorporated hereinby reference, in their entirety. Aspects of the implementations can bemodified, if necessary to employ concepts of the various patents andapplications to provide yet further implementations.

These and other changes can be made to the implementations in light ofthe above-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificimplementations disclosed in the specification and the claims, butshould be construed to include all possible implementations along withthe full scope of equivalents to which such claims are entitled.

What is claimed is:
 1. A variable focal length (VFL) lens systemcomprising: a tunable acoustic gradient (TAG) lens operated toperiodically modulate its optical power over a range of optical powersat an operating frequency; and an optical focus monitoring configurationfor monitoring a focus state of the VFL lens system, the optical focusmonitoring configuration comprising: a monitoring light sourceconfigured to input focus detection light into the TAG lens during theperiodic modulation, wherein: the input focus detection light isconfigured to provide an input amount of light energy distributed in aninput illumination pattern having an approximately constant size; and atleast a central portion of the input illumination pattern is transmittedthrough the TAG lens during the periodic modulation to provide acorresponding output illumination pattern from the TAG lens, the outputillumination pattern having a size and intensity that depends on theoptical power of the TAG lens; and an optical focus signal detectorportion positioned at an approximately constant distance from the TAGlens to receive focus detection light included in the outputillumination pattern output from the TAG lens, the optical focus signaldetector portion comprising a filtering configuration and a focusphotodetector that provides a focus output signal that varies inrelation to a total light energy that the focus photodetector receives,wherein the filtering configuration receives the output illuminationpattern and limits an amount of the included focus detection light thatreaches the focus photodetector and a focus monitoring signal isprovided based on the focus output signal provided by the focusphotodetector.
 2. The VFL lens system of claim 1, wherein the focusdetection light is at least approximately collimated in the inputillumination pattern.
 3. The VFL lens system of claim 1, wherein theinput illumination pattern is configured such that the correspondingoutput illumination pattern comprises a solid pattern, and the filteringconfiguration comprises a spatial filtering aperture that is defined bylimits of the focus photodetector, and the solid pattern overfills thefocus photodetector at all times during the periodic modulation.
 4. TheVFL lens system of claim 1, wherein: the filtering configurationincludes a mask comprising a spatially filtering aperture; the maskblocks a blocked portion of the output illumination pattern at all timesduring the periodic modulation; the spatially filtering aperturetransmits a transmitted portion of the output illumination pattern atall times; and the spatially filtering aperture is shaped such that aratio of the transmitted portion to the blocked portion varies dependingon the size of the output illumination pattern.
 5. The VFL lens systemof claim 4, wherein the input illumination pattern is configured suchthat the corresponding output illumination pattern comprises an annularpattern.
 6. The VFL lens system of claim 1, wherein the filteringconfiguration includes a mask comprising a density filter having anon-uniform density pattern configured to attenuate the transmission ofthe output illumination pattern depending on the size of the outputillumination pattern.
 7. The VFL lens system of claim 6, wherein thedensity filter is configured to receive and transmit the entire outputillumination pattern to the focus photodetector at all times during theperiodic modulation.
 8. The VFL lens system of claim 6, wherein theinput illumination pattern is configured such that the correspondingoutput illumination pattern comprises an annular pattern, and thenon-uniform density pattern is axisymmetric and a density of thenon-uniform density pattern varies as a function of radius within theoutput illumination pattern.
 9. The VFL lens system of claim 1, whereinthe periodic modulation corresponds to a frequency of at least 50 kHz,the focus monitoring signal comprises a time varying signal that isindicative of the focus state of the TAG lens throughout the modulationperiod, and the time varying signal is provided with a latency comparedto the focus state of not more than 100 nanoseconds.
 10. The VFL lenssystem of claim 1, comprising a controller that operates to drive theTAG lens at a resonant frequency in order to periodically modulate theTAG lens optical power over the range of optical powers at the operatingfrequency, wherein the focus monitoring signal is input to thecontroller and is used to adjust at least one of amplitude, frequency,or phase of the periodic modulation of the TAG lens.
 11. The VFL lenssystem of claim 1, further comprising an imaging configurationcomprising the TAG lens, an objective lens, and a camera portion,wherein: the objective lens inputs workpiece light from an imagedsurface region of a workpiece in a field of view (FOV) of the imagingconfiguration and transmits the workpiece light through the TAG lens,and the camera portion receives the workpiece light from the TAG lensand provides an image exposure such that it is focused at acorresponding imaging system focal plane having at least one of a focusdistance or Z-height relative to the imaging configuration; at least oneof the focus distance or Z-height of the imaging system focal plane iscontrolled by the TAG lens optical power; and the focus monitoringsignal is indicative of at least one of the focus distance or Z-heightof the imaging system focal plane.
 12. The VFL lens system of claim 11,wherein the VFL lens system includes calibration data that relatesrespective focus distances or Z-heights to respective focus monitoringsignal values.
 13. The VFL lens system of claim 11, wherein: the VFLlens system is configured to control the image exposure using an imageexposure timing that determines the corresponding imaging system focalplane; and the VFL lens system is configured to control at least one ofa timing of a controllable strobe light source that is included in theVFL lens system or a timing of a controllable image integration periodof the camera portion, to provide the image exposure timing.
 14. The VFLlens system of claim 13, further comprising a latching circuitconfigured to latch a focus monitoring signal value at a timecorresponding to the image exposure timing, wherein the latched focusmonitoring signal value is indicative of the focus distance or Z-heightfor the corresponding image exposure.
 15. The VFL lens system of claim13, further comprising a comparator circuit configured to input thefocus monitoring signal and input a reference signal related to adesired imaging focus distance or Z-height, and output a trigger signalthat controls the image exposure timing to occur when the focusmonitoring signal corresponds to the reference signal.
 16. The VFL lenssystem of claim 11, wherein the monitoring light source is configured toprovide focus detection light consisting of a first set of wavelengthsin the input illumination pattern, and the optical focus monitoringconfiguration further comprises: a first beamsplitter that is locatedbetween the objective lens and the TAG lens and receives focus detectionlight from the monitoring light source and directs the inputillumination pattern to pass through the TAG lens along with theworkpiece light; and a second beamsplitter that is located between theTAG lens and the camera portion wherein the second beamsplitter isconfigured to receive the output illumination pattern from the TAG lensalong with the workpiece light and reflect the first set of wavelengthsincluded in the output illumination pattern toward the optical focussignal detector portion and transmit other wavelengths included in theworkpiece light to the camera portion.
 17. The VFL lens system of claim1, wherein: the optical focus monitoring configuration further comprisesa beamsplitter that directs a first split output illumination pattern tothe optical focus signal detector portion and directs a second splitoutput illumination pattern to a normalization optical detectorconfigured to transmit the entire second split output illuminationpattern to a normalization photodetector that provides a normalizationoutput signal that varies in relation to the total light energy that thenormalization photodetector receives; and the VFL lens system furthercomprises a focus monitoring output circuit that is configured to inputthe focus output signal and the normalization output signal and toproduce a normalized focus monitoring signal, wherein variations in thefocus output signal due to variations in the total light energy includedin the output illumination pattern are compensated based on thenormalization output signal.
 18. The VFL lens system of claim 1, whereinthe monitoring light source comprises a pattern generator thatdetermines a shape for the input illumination pattern that is input intothe TAG lens, resulting in a corresponding shape for the outputillumination pattern from the TAG lens.
 19. The VFL lens system of claim1, wherein the input amount of light energy distributed in the inputillumination pattern is approximately constant.
 20. A method foroptically monitoring a focus state of a variable focal length (VFL) lenssystem comprising a tunable acoustic gradient (TAG) lens, in order toprovide a focus monitoring signal that reflects the focus state withhigh accuracy and without significant latency, the method comprising:operating the TAG lens to periodically modulate its optical power over arange of optical powers at an operating frequency; inputting a focusdetection light into the TAG lens during the periodic modulation,wherein the input focus detection light is configured to provide aninput amount of light energy distributed in an input illuminationpattern having an approximately constant size; transmitting at least acentral portion of the input illumination pattern through the TAG lensduring the periodic modulation to provide a corresponding outputillumination pattern from the TAG lens, the output illumination patternhaving a size and intensity that depends on the optical power of the TAGlens; receiving focus detection light included in the outputillumination pattern using an optical focus signal detector portionpositioned at an approximately constant distance from the TAG lens, theoptical focus signal detector portion comprising a filteringconfiguration and a photodetector that provides a focus output signalthat varies in relation to the total light energy it receives, whereinthe filtering configuration receives the output illumination pattern andlimits the amount of included focus detection light that reaches thephotodetector; and providing a focus monitoring signal based on thefocus output signal provided by the photodetector.
 21. The method ofclaim 20, wherein the focus detection light is at least approximatelycollimated in the input illumination pattern.
 22. The method of claim20, wherein: the filtering configuration includes a mask comprising aspatially filtering aperture; the mask blocks a blocked portion of theoutput illumination pattern at all times during the periodic modulation;the spatially filtering aperture transmits a transmitted portion of theoutput illumination pattern at all times; and the spatially filteringaperture is shaped such that a ratio of the transmitted portion to theblocked portion varies depending on the size of the output illuminationpattern.
 23. The method of claim 20, wherein the filtering configurationincludes a mask comprising a density filter having a non-uniform densitypattern configured to attenuate the transmission of the outputillumination pattern depending on the size of the output illuminationpattern.
 24. The method of claim 20, wherein the periodic modulationcorresponds to a frequency of at least 50 kHz, the focus monitoringsignal comprises a time varying signal that is indicative of the focusstate of the TAG lens throughout the modulation period and the timevarying signal is provided with a latency compared to the focus state ofnot more than 100 nanoseconds.
 25. The method of claim 20, wherein: theVFL lens system further comprises an imaging configuration comprisingthe TAG lens, an objective lens, and a camera portion; the objectivelens inputs workpiece light from an imaged surface region of a workpiecein a field of view (FOV) of the imaging configuration and transmits theworkpiece light through the TAG lens, and the camera portion receivesthe workpiece light from the TAG lens and provides an image focused atan imaging system focal plane having at least one of a focus distance orZ-height relative to the imaging configuration; at least one of thefocus distance or Z-height of the imaging system focal plane iscontrolled by the TAG lens optical power; and the focus monitoringsignal is indicative of at least one of the focus distance or Z-heightof the imaging system focal plane.